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InfoSec Notes

Validating certificate chains with openssl

Using openssl to verfiy certificate chains is pretty straight forward – see a full script below.

One thing that confused me for a bit was how to specify trust anchors without importing them to the pki config of the os (I also did not want to accept all of the trust anchors).

So.. here what to do for specif trust anchors

# make a directory and copy in all desired trust anchors
# make sure the certs are in pem format, named <bah>.pem
mkdir ~/trustanchors
# create softlinks with hash 
cd ~/trustanchors
for X in ./*.pem;do ln -s $X ./`openssl x509 -hash -noout -in $X`.0;done

# confirm the trust anchor(s) are working as expected
openssl verify -CApath ~/trustanchors -CAfile <some_intermediate>.pem <my_leaf>.pem

So here’s a simple script that will pull the cert chain from a [domain] [port] and let you know if it is invalid – note there will likely be come bugs from characters being encoded / return carriagesĀ missing:

#!/bin/bash

# chain_collector.sh [domain] [port]
# output to stdout
# assumes you have a directory with desired trust anchors at ~/trustanchors

if [ $# -ne 2 ]; then
	echo "USAGE: chain_collector.sh [domain] [port]"
	exit 1
fi

TRUSTANCHOR_DIR="~/trustanchors"
SERVER=$1:$2
TFILE="/tmp/$(basename $0).$$.tmp"
OUTPUT_DIR=$1_$2
mkdir $OUTPUT_DIR

openssl s_client -showcerts -servername $1 -connect $SERVER 2>/dev/null > $TFILE
awk 'BEGIN {c=0;} /BEGIN CERT/{c++} { print > "tmpcert." c ".pem"}' < $TFILE 
i=1 
for X in tmpcert.*.pem; do
    if openssl x509 -noout -in $X 2>/dev/null ; then 
        echo "#############################"
        cn=$(openssl x509 -noout -subject -in $X | sed -e 's#.*CN=\(\)#\1#')
	echo CN: $cn
	cp $X $OUTPUT_DIR/${cn// /_}.$((i-1)).pem 
	cert_expiry_date=$(openssl x509 -noout -enddate -in $X \
			| awk -F= ' /notAfter/ { printf("%s\n",$NF); } ')
	seconds_until_expiry=$(echo "$(date --date="$cert_expiry_date" +%s) \ 
                                     - $(date +%s)" |bc)
        days_until_expiry=$(echo "$seconds_until_expiry/(60*60*24)" |bc)
	echo Days until expiry: $days_until_expiry
	echo $(openssl x509 -noout -text -in $X | \ 
                grep -m1 "Signature Algorithm:" | head)
	echo $(openssl x509 -noout -issuer -in $X)
	if [ -a tmpcert.$i.pem ]; then
		echo Parent: $(openssl x509 -noout -subject \ 
                                   -in tmpcert.$i.pem | sed -e 's#.*CN=\(\)#\1#')
	        echo Parent Valid? $(openssl verify -verbose -CAfile tmpcert.$i.pem $X)	
	else
		echo "Parent Valid? This is the trust anchor"
	fi
	echo "#############################"
    fi
    ((i++))
done
rm -f tmpcert.*.pem $TFILE

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